The developing trend of the DC/DC converters is in the same direction like most of the power supply products that is to have the high efficiency, the high power density, the high reliability and the low cost. Since the resonant converters usually employ the controlling method of pulse frequency modulation (PFM), the zero-voltage switching of their switches is relatively easy to accomplish so as to achieve the requirements of high efficiency and high power density.
However, the resonant converters still have some problems when simply adopting PFM during the abnormal status such as a circuit starting status or an output current-limiting status etc. A relatively lower voltage gain of the circuit is required when a resonant converter is working under an abnormal status; especially when a circuit is under the output current-limiting status, the circuit current has to be controlled so as to cause the circuit to operate normally. To achieve these requirements, the working frequency of the resonant converter has to be increased, and the increased working frequency means that the loss of the circuit is also increased. The total loss of the converter is larger than the converter could bear when the frequency increased to a specific value, and the converter is thus damaged. Please refer to FIG. 1, which shows a waveform diagram of a converter's frequency ratio vs. the converter's duty ratio regarding a controlling method of simply adjusting the frequency for a resonant converter in the prior art. In FIG. 1, the horizontal axis is the duty ratio of the converter and the vertical axis is the frequency ratio of the working frequency of the converter to the resonant frequency of the converter.
The aforementioned control method of simply changing the frequency cannot solve the problem that the loss of the circuit is too big to bear when it is working under an abnormal status. Another controlling method uses the frequency modulation plus the pulse-width modulation as shown in FIG. 2. Referring to FIG. 2, it shows a waveform diagram of a converter's frequency ratio vs. the converter's duty ratio regarding a controlling method of adjusting the frequency firstly and the duty ratio secondly, for a resonant converter in the prior art. Also, the horizontal axis is the duty ratio of the converter and the vertical axis is the frequency ratio of the working frequency and the resonant frequency of the converter. The working trace of the converter goes by controlling the duty ratio from A to B firstly, and then modifying the operating frequency of the converter within BC stage secondly, and vice versa. Due to that the controlling method of pulse-width modulation is employed, the switch frequency is relatively lower than that only adopting of the controlling method of changing the frequency under the same voltage gain. But through minishing the pulse-width, it may cause the hard-switching of the circuit which increases the total loss of the converter.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a resonant converter system and a controlling method thereof having a relatively better efficiency. In FIG. 3, it shows a waveform diagram of a converter's frequency ratio vs. the converter's duty ratio regarding a controlling method of adjusting the frequency and the phase angle for a resonant converter according to the preferred embodiments of the present invention.